Spectrum analyzers use intermediate frequency (IF) bandpass filters to resolve signals of different frequencies. These filters have variable bandwidths. Two types of analog electronic filters are commonly used in combination to build these IF filters: crystal and LC. The crystal filters provide a range of bandwidths that are narrower than the bandwidths provided by the LC filters.
In spectrum analyzer IF filters, the crystal filters are used in series resonant mode, while the LC filters are used in parallel resonant mode. Parallel resonance provides anti-resonance, a condition of maximum impedance at a range of basic frequencies. Series resonance provides resonance, a condition of low impedance at a range of frequencies. The basic topology used in the prior art for LC filters is shown in FIG. 1 and the basic topology used in the prior art for crystal filters is illustrated in FIG. 2. In both topologies, the variable resistors determine the bandwidth of the filters but also varies the passband gain of the filter.
Crystal filters have several drawbacks. One major problem is that the high input impedance amplifier, such as a FET-input buffer amplifier, which buffers the output of the crystal filter, has a relatively high input noise voltage. The high input noise voltage causes the noise figure of the filter to be higher than desirable. Another problem is that the narrowest bandwidth setting requires more drive current than the wider bandwidths because the input resistance of the IF filter is lowest in the narrowest bandwidth. The widest bandwidth setting has the highest resistance and the resulting noise voltage of the variable resistor can aggravate the noise figure further. A third problem is that crystals have nonlinear properties which can cause shifts in crystal resistance and frequency at high levels of current. These shifts cause the gain to change with signal level and decrease the amplitude accuracy of the filter. Furthermore, crystals manufactured to minimize these shifts are expensive.
LC filters also have limitations. In contrast to the crystal filters, the narrowest bandwidth setting has the highest impedance, the resulting noise voltage may aggravate the noise figure. Secondly, the widest bandwidth setting requires more drive current because the input resistance is low. In order to avoid excessive losses in IF filters, low loss (high-Q) crystals and inductors are used. High-Q crystals and inductors are very expensive to build. There is a conflict between the need to not have gain variations caused by losses and the desire to use lower-Q, less expensive parts. The filter losses due to Q change as the bandwidth of the filters is changed. Precision filtering applications require that the effect of these gain variations with respect to bandwidth be removed.
Prior art addressed these gain variations in crystal filters in several ways. One method to resolve this design conflict is to allow the loss of the filter to vary and change the gain of a preamplifier or post-amplifier to cancel the loss of the crystal filter. Another prior art solution uses a negative output impedance buffer amplifier to cancel the crystal motional resistance. FIG. 3 shows a third prior art solution, disclosed by Viaden Temer in U.S. Pat. No. 4,600,903, in which a feed forward transformer compensates for the loss in the voltage divider from by the crystal motional resistance and the resistors R2 and R3.
These prior art solutions do not provide constant gain throughout the bandwidth range while still providing a low noise figure. Nor do these prior art solutions share common amplifiers, variable resistors, or inductors among the crystal and LC filters. An intermediate frequency bandpass filter having nearly constant gain with fewer components, including inductors, crystals, variable resistors, and amplifiers, is desirable.